Internal combustion engines may operate in a variety of combustion modes. One example mode is homogeneous charge compression ignition (HCCI) also known as controlled autoignition (CAI), wherein a charge including an air and fuel mixture is compressed until autoignition occurs without necessarily requiring a spark being performed by a sparking device. In some conditions, HCCI may have greater fuel efficiency and reduced NOx production compared to other combustion modes such as spark ignition. However, HCCI may be limited under some conditions, since autoignition may be at least partially limited by the temperature of the charge and/or compression performed within the cylinder. Further, transitions into and out of the HCCI operation may present various control issues.
One approach to enable operation in a variety of combustion modes is described in U.S. 2005/0183693. In this example, a cylinder with cam profile switching is used to selectively provide operation in HCCI and SI modes.
However, the inventors herein have recognized a disadvantage with such an approach. For example, during the transient conditions of SI/HCCI transitions in the above system, the cylinders may receive unsuitable relative intake air amounts between different temperature air streams, such as more air of an increased temperature than needed for some operating conditions. Further, the approach of conditioning intake air, for example, by heat addition may provide degraded temperature control of the charge temperature during combustion mode transitions, which may be caused by operation in a particular mode.
In one approach, the above issues may be addressed by a method of operating an engine having at least one cylinder, the method comprising directing a first air stream to an intake valve of the cylinder; directing a second, separate air stream to a second intake valve of the cylinder, said second air stream at a higher temperature than said first air stream; operating said first intake valve and adjusting at least one of an opening timing and a closing timing of said first intake valve to adjust engine output; and intermittently operating said second intake valve to maintain said higher temperature of said second air stream.
In this way, by utilizing at least one electrically actuated valve to intermittently leak intake air into the cylinder from the heated intake passage, a ready reserve of heated air may be achieved prior to a transition to HCCI operation, while also reducing degradation of engine performance during spark ignition operation.
The inventors have also recognized that some operating modes may utilize differing amounts of the conditioned (e.g. heated or cooled) intake air. As such, energy used to facilitate heating and/or cooling of the intake air may be wasted. Further, the extent to which the intake air is conditioned may also affect the charge temperature control. For example, as the amount of heating provided to the intake air is reduced the charge temperature may vary.
In another approach, the above issues may be addressed by a method of operating an engine having at least one cylinder, at least one intake passage operatively coupled to the cylinder for directing intake air to the cylinder, and a heat exchanger arranged along the intake passage to heat the intake air passing through the intake passage before reaching the cylinder, the method comprising: varying an amount of heat provided to the intake air by the heat exchanger in response to a combustion mode of the cylinder; and supplying heated intake air to the cylinder by intermittently operating an intake valve, wherein said intermittent operation of the intake valve is varied in response to the amount of heat provided to the intake air by the heat exchanger.
In this way, the amount of heating provided to the intake air may be adjusted based on the amount of heated air used during the particular operating mode and the operation of an intake valve controlling the delivery of the heated air to the cylinder may be adjusted to maintain the desired charge temperature.